Fundamentally, compressors, gas turbines, steam turbines and other thermal machines are subjected to high thermal and mechanical stresses. Accordingly, it is indispensable to reduce such thermal and mechanical stresses.
In a gas turbine, a rotor shaft, among the various other parts, such as rotor blades and stator vanes, are exposed to high thermal and mechanical stresses. Critical locations may be, among others, cooling bore inlets in rotor cavities of the rotor shaft. Generally, the rotor cavities are configured inside of the rotor shaft, and the cooling bore inlets are arranged on outer circumference of such rotor cavities. The cooling bores extend from the inside of the rotor shaft mainly in a radial direction. Where such cooling bores and rotor cavities are concerned, the stresses arising in the rotor cavities depend critically on a cross-sectional contour of the rotor cavities.
The cooling bores usually constitute a mechanical weakening of the rotor shaft in the area where they extend from the rotor cavities, which may have an adverse effect in the case of high thermal and mechanical stresses.
Accordingly, there are a number of measures which have already been contemplated to reduce the effects of thermal and mechanical stresses, namely:
Reduction of the bore diameters and change of the bleed position within the compressor for realize a higher stage. But this impact increases the cooling air pressure and thus reduces the required cross section of the flow. Referring to the drawbacks this induces a negative performance impact and, additionally, this increases the cooling air temperature.
Change of the SAF system, e.g. change of the blade feed to the front of the blade instead of the bottom. Referring to the drawbacks this requires a redesign of the rotor and/or rotor blades and/or stator vanes. Additionally, the pressure losses must be recuperated with other setups.
The internal radial compressor of the rotor is provided in the form of ribs on the rotor cavity wall. The internal ribs accelerate the air flow in circumferential direction and thus increase its swirl. Referring to the drawbacks this comports that the ribs have a very high surface to volume ratio and thus have a very fast thermal behaviour while the rotor disc with a very low surface to volume ratio has a very slow thermal behaviour. This can introduce very high thermal stresses into the rotor disc so that the design of such ribs results difficult.
In summary it can be said that a high number of big holes lead to limited rotor lifetime due to a low remaining wall thickness between neighboring bores. Furthermore, the high jump of the relative velocity of the cooling air at the inlet of the cooling bore leads to pressure losses and a bigger required bore diameter due to recirculation.